The present invention relates to rigid mirrors of the type called SSMs ("Second Surface Mirrors"), which can be used on the panels of spacecraft, particularly artificial satellites, for the purpose of heat control.
More particularly, the invention concerns a method and a unit for mounting an array of SSMs on a panel of a spacecraft, particularly an artificial satellite.
The internal temperature of an artificial satellite must be kept under control in order to ensure the perfect efficiency of the apparatus within the satellite.
The methods used until now for controlling the temperature of spacecraft, and particularly artificial satellites, make use of active systems and passive systems. The active systems employ fairly complicated and delicate devices (cooling systems, thermostats, etc.) which, although ensuring more precise control of the temperature than passive systems, prejudice the reliability and the lightness of the satellite.
It is generally preferred, therefore, to use passive systems which ensure temperature control of the spacecraft by maintaining a strict equilibrium between the incident radiant energy absorbed by the outer surface of the spacecraft from radiant energy sources in space, and the energy emitted by the spacecraft and the energy reflected into space. In other words, the energy emitted into space must be equal to the sum of the energy generated in the form of heat by the equipment within the spacecraft and the energy from radiant energy sources in space.
These passive systems make particular use of radiant surfaces which are able to reflect heat from the exterior and transmit heat from the interior of the vehicle and generated by apparatus with which it is equipped.
Various types of radiant surfaces have been made in the past for controlling the temperature of spacecraft. In general, in order to form these surfaces, it is necessary to achieve a compromise between the need to provide very good optical properties and the need to ensure a prolonged working life. These requirements do, in fact, oppose each other in that a very satisfactory optical surface is also easily degradable and is exposed to the risk of damage as a result of bombardment by the various types of radiation to which the surface is subject when in space.
An optimum solution which is able to satisfy both these needs effectively is constituted by rigid mirrors of the so-called SSM ("Second Surface Mirror") type. Typically, one is dealing with mirrors constituted by rectangular or square sheets of glass generally having a size of between 400 mm.sup.2 and 1600 mm.sup.2 and a thickness of 150-200 microns. The glass is coated on one surface with a silver film having a thickness of 100-1000 Angstroms. In more recent types, the mirror is also coated overall with a layer of tin and indium oxide. This layer of electrically conductive material ensures that the electric charges which tend to accumulate on the mirrors when the satellite is in space are discharged to the metal mass of the satellite panel which supports the mirrors. In fact, these SSM mirrors constitute surfaces with very high resistivities which may accumulate a high electrical potential. The electrical charges accumulated on the SSMs may become so high as to give rise to an arc discharge from the surfaces of the mirrors to the underlying metal surface of the spacecraft, and also to discharges between adjacent mirrors. In order to avoid this effect, it is necessary to ensure an electrical connection between the various mirrors and between the mirrors and the metal panel which supports them.
A number of requirements must be taken into account in the application of the SSMs to a panel of a spacecraft:
First of all it is necessary, for the reasons explained above, to ensure an electrical connection between the various mirrors themselves and between the various mirrors and the panel of the spacecraft; in the second place the mirrors must be located precisely, ensuring that the plane of the mirror is as parallel as possible to the plane of the panel. If this were not the case, adjacent mirrors would not be coplanar, with disadvantages with regard to the characteristics of emissivity of the radiant surface. Finally, there is a need, no less important than those indicated above, to achieve the application of the mirrors in a simple and quick manner.
Until now, to the knowledge of the Applicants, the only existing method for the mounting of an array of mirrors of the type indicated above on a panel of a spacecraft consists of fixing each mirror manually to the panel by gluing. On the other hand, if one takes into account the fact that an artificial satellite may be equipped with a very large number of mirrors (a surface of about 8 m.sup.2, for example, corresponds to about 10,000 mirrors), it is clear that the manual method used up to now involves very high labour costs and time.
To the knowledge of the applicants, however, no one has succeeded until now in achieving a method for the rapid mounting of SSMs on a panel of a spacecraft, which ensures that the necessary characteristics of planarity, cleanness of the surfaces, effectiveness of adhesion, and electrical connection to earth are obtained.